Method of avoiding coarse crystallization at the solidification of metallic castings



Patented June 2,1936

UNITED STATES warrior) or AVOIDING coansa cars'rsn- LIZATION AT THE METALLIC CASTINGS SOL IDIFICATION F Erich .Scheil, Dortmund, Germany, assignmto firm Vereinigte Stahlwerke Dusseldorf, Germany No Drawing. Application April 29, 1933,8erlal. No. 668,671. In Germany May 2, 1932 "21 Claims. (01. 75-93) Crystallization of a metal or alloy} is known to proceed from single points called nuclei. During the solidification of the metal or alloy the number of these nuclei determines the'size of the grain. The number of existing and spontaneously developing nuclei is so small in many alloys that a coarse grain is formed. 'The' coarse grain reduces the capacity of the alloy for'shaping without scraping 'or renders it even impossible [and Otherwise also jures the mechanical properties of alloys,

The measures generally employed against this coarseness of grain are accelerated cooling and low pouring 'temperature. The first measure is of course efiective only atsmall cross sections of ingots. The efiectiveness of the second measure does not sufilce in all cases. Furthermore, the carrying out of this measure, particularly when casting large units, involves considerable difficulties.

According to the invention, coarseness of grain is reduced by artificially increasing the nuclei present in the liquid metal by,additions, so that the structure of the ingot will become fine-grained and the working thereof by forging, rolling, draw-v ing or similar deforming processes will be facilitated or made possible. The production of these nuclei can, as a matter of principle, take place by means of various substances. Most effective is the addition of such kinds of crystals which are equal to those pro duced at the'crystallization of the melt or isomorphous. i

In the simplest way this increase of nuclei can be effected by the addition of-solid pieces of the' same alloy to the melt. Addition ought to take place after the finishing of the metallurgical operations such as alloying, deoxidizing, degasifymg, or the like. These pieces which might be referred to as generators of nuclei must dissolve in the melt to such an extent that after solidification substantially no unmelted parts can be noted, since that would cause defects in the casting. 1 There will remain then in the melt very many much more dimcultly destroyable nuclei which grow into crystals duringsolidification'and thus produce a fine grain. The finergrained the material of the nucleus generator the more nuclei will remain. Gradually, however, these nuclei will melt too, so that the number of nuclei will be the greater the shorter the time is between addition of the nucleus generator and. soliidification. If the melting speed alloy,'for any reasons, cannot be-chosen short enoughto attain a grain of sumcient fineness, the addition alloy may slightly deviate from the composition of the melt so as to have a higher melt- 'ing point being approximately 50 C. above the melting pointof the main alloy.

Harmful destruction of nuclei will be the smaller the lower the temperature -of the melt is at the addition of the nucleus generator. From this, regulation of the pouring-temperature. can be derived without trouble. Itshould be as low as possible, though still high enough to insure perfect dissolution of the nucleus carriers. rule, a change of the normal casting temperature is unnecessary. 1

Any motion of the melt after the addition of Asa the nucleus generator will have a destructive effect upon the nuclei also. Motions will be slightestwhen the nucleus generator is inserted into the mould (chill or sand mould) and molten only upon the pouring in of the liquid metal to be cast leaving the desired residual nuclei. For example, the nucleus generator may be secured in the mould in the form of a rod or wire. The number of rods and their thickness should be chosen so that the motion always occurring in the melt in the mould sufilces for the uniform distribution of the nuclei, which should be ascertained by simple experiments as the case arises.

The invention may be applied for instance to steel or zinc melts in the following manner:

possible almost completely to obviate said'transcrystallization by inserting 4 wires of a thickness of 1 mm. into a 20 kg. melt which was cast into a round chill mould of a bar cross-section pl m In the neighbourhood of the cooling wall the thickness of the rods should probably be chosen somewhat less. The thickness of the rods of the nucleus generator should-be adapted to the wall thickness of the ingot in such away that it will bethe greater the greater the cross section of the ingot, since otherwise the nucleus generators would be destroyed too quickly and,

pieces thereof into the casting jet.

approximately near 0.1% of the casting. How-' ever, the amount of nucleus material may be much higher and, for instance, be as high as about 5%.

If the materialof the nucleus generator corrodes easily or forms scale vto a considerable degree during heating, so that at contact with the melt gases or at least flaws develop, it may be provided with a protective covering of a known kind protecting the inserted rods or the like against corrosion andscallng.

Instead of. arranging the nucleus generator in the mould, it may be added by throwing small Both methods, that is, arrangement of the nucleus generator in the mould and throwin it into the jet or additionto the ladle, may be combined. It will ,thenbe advantageous to inn crease the addition to the" jet towards the end of the casting operation. It is further advisable to provide for a larger diameter on top of the ingot than below in order to avoid piping.

In the foregoing it has been stated that the nucleus generators are to be added in the, form or pieces which areadapted to be substantially destroyed by the available heat, that is, tosuch an extent that only'small yet numerous remnants are left in the melt. Said remnants will be of miscroscopic or sub-microscopicsize which .is sufiicient for forming nuclei. The exact shape and dimensions of the inserted substances which .are suitable for each purpose can easily be determined in practiceby a simple test. For this purpose preliminary test bars of different thickness are inserted and the variation in thickness is continued until the microscopic inspection shows a complete or a nearly complete melting of the during the solidification of the ingots.

ingots.

added substance, so that the nucleus generator is present only in microscopic or sub-microscopic parts. 4

What I claim and'desire to secure by Letters Patent in the UnitedState's is: y

1. Method of obtaining a fine crystallization of metallic castings in order to enhance their workability which consists in adding to the liquid metal several solid pieces'of the same alloy as the cast material, the size of, said pieces being determined in accordance with the transcrystallization tendency of the metal in such a manner that they are completely molten before, the casting ingots begin to solidify, 1but leaving numerous very small invisible remnants forming nuclei 2 Method of obtaining a fine crystallization of metallic castings in order to enhance their workability which consists in adding to the liquid metal several solid pieces of a metal or metal alloy of an isomorphous crystalline structure having a melting point up to C. higher than .the metal to be cast, the size of said pieces being determined in accordance with the transcrystallization tendency of the metal in such a manner that they are completely molten before the casting ingots begin tb solidify, but leaving numercus very small invisible remnants forming nuclei during the solidification of the ingots.

3. Method of obtaining a fine crystallization of metallic castingsin order to enhance their workability which consists in inserting into the mould several barlike solid pieces of a metal or metal alloy of a fine grained material and of a structure at least isomorphous to that of the metal to be cast, the size of said pieces being adapted to the transcrystallization tendency of the-metal to be cast in such a manner that they are completely' molten before the casting ingots begin to solidify and securing them therein'prior to the casting operation, thereupon pouring in the metal to be cast, thereby melting the inserted pieces from the bottom upwards in accordancewith the rise in level at the liquid to such an extent that only numerous very small invisible remnants are left in the melt of a sizeisumoient for forming nuclei anion scrrnm during the solidification of the I 

